What’s the Story on Vitamin D?

“Triple That Vitamin D Intake, Panel Prescribes,” was the headline in the Wall Street Journal.

“Extra Vitamin D and Calcium Aren’t Necessary, Report Says,” the front page of the New York Times proclaimed that morning about the same much-­anticipated 1,000-page report from the influential Institute of Medicine (IOM), which devises the recommended nutrient intakes for the U.S. and Canadian governments, among other tasks.

Which head­line was right? They both were, to a large extent, which shows just how confusing the report is—and why many people don’t know whether they should start, or stop, taking these supplements.

For years, many experts have complained that the 1997 IOM guidelines about vitamin D intake were too low. Since then, there has been a lot of research on vitamin D. Some has focused on its known benefits, notably its crucial role in working with calcium to keep bones strong.

But many studies have linked high blood levels of the vitamin to a reduced risk of everything from heart disease, multiple sclerosis, and some cancers to diabetes, infections, hypertension, and falls caused by age­-related muscle weakness. The research has sounded so promising that sales of vitamin D pills have been skyrocketing. Many doctors have been testing their patients’ blood levels of vitamin D and advising supplements for those who score low. The new report will probably slow the D bandwagon.

D-tails from the report

The panel’s guidelines pertain only to vitamin D’s well-­established role in bone health and fracture reduction. It concluded that the evidence for all other benefits was “inconsistent and/or inconclusive or did not demonstrate causality.”

The new recommended intake is 600 international units, or IU, a day through age 70 (previously, it was 200 IU up to age 50, and 400 IU between 50 and 70). For people over 70, the new goal is 800 IU (up from 600 IU). So for younger people, the number has indeed tripled. It’s a little like comparing apples and oranges, though, since the 1997 recommendations were for what’s called “adequate intakes,” and the new ones are for the better-­known “recommended dietary allowances,” or RDAs, which are usually higher. The panel also doubled the safe “upper level intake” to 4,000 IU a day.

Many prominent vitamin D experts were disappointed by the new guidelines, which they consider too low, and still recommend supplements of 1,000 to 3,000 IU a day. And they point to evidence that it’s safe to take as much as 10,000 IU a day.

Keep in mind, vitamin D is not found in many foods besides fortified milk (100 IU per cup) and some fish, though increasingly it is being added to other foods. That’s why many experts— including several osteoporosis groups and the Canadian Cancer Society—as well as the Wellness Letter, have advised people who don’t get enough D from food to take a supplement.

Forget about the sun

Complicating matters for estimating vitamin D needs is the fact that D is a hormone that your skin manufactures when exposed to sunlight and that your body regulates in various ways. The amount you produce depends on the time of day, season, how far north you live, skin pigmentation (darker skin makes less D), how much of your body is exposed to the sun, and your age (older people produce less). The vitamin is fat­-soluble, so your body can store it for the days, or even months, when you don’t get any sun or consume any D.

Because D production in the skin is so variable, and because excessive sun exposure increases the risk of skin cancer, the IOM panel’s recommendations assume that people get little or no vitamin D from the sun. Obviously, though, people do produce some D from even incidental sun exposure.

Surprises and controversies

So was the Times headline wrong in reporting that the IOM said that we don’t need more D? No, the panel did conclude that almost all people are meeting their vitamin D needs, though it left open the option of supplements for some groups.

But that runs counter to much recent research—based either on nutrient intake surveys or blood levels of D—which suggests that many, if not most, people don’t get enough D, especially those who are older, have darker skin, or live in the northern U.S. or Canada (where the sun is too weak to create D much of the year). The preliminary 2010 Dietary Guidelines for Americans cites re­search showing that most people do not meet even the old low recommended intakes for vitamin D and may be deficient.

One crucial issue is that the IOM chose a lower-than-expected cut­off point for adequate blood levels of vitamin D. In recent years, most experts and testing labs have said blood levels should be at least 30 ng/ml (nanograms per milliliter of blood) of 25­-hydroxyvitamin D for optimal bone health; most people fall short of that.

Some researchers believe that 40 to 60 ng/ml is a better goal, especially for other proposed benefits. The IOM panel concluded that 20 ng/ml (equal to 50 nanomoles per liter, as measured in Canada) is sufficient for bone health—which is hotly contested—and that nearly all Americans and Canadians are above that. In any case, the panel said that blood tests are unreliable and the results are not standardized. That leaves the whole concept of measuring vitamin D levels up in the air.

The panel’s dismissal of hundreds of studies suggesting that vitamin D has benefits beyond bone health rankled many experts in the field. It’s true that nearly all of this research has been lab or observational studies, not the large, long­term clinical trials that can prove cause and effect, and that some of the results have been conflicting. But the report seemed to take seriously the few studies suggesting that high daily intakes or high blood levels of D may be harmful. “Why ignore the vast majority of these studies, which point in the direction of benefit, and focus on the one in a hundred that indicated some increased risk?” asked Edward Giovannucci, M.D., of Harvard, a leading researcher on vitamin D.

What to do

The IOM’s dietary recommendations are always cautious and risk-­averse, since they are aimed at the entire population. Several large, rigorous clinical trials are underway to test the various health claims made for higher intakes of D, but the results won’t be in for years. Meanwhile, you may wish to follow the IOM panel’s advice, especially if you want to be cautious and if you consume lots of milk and other D-fortified foods, get some sun exposure (but not too much), and don’t like to take supplements.

If, however, you want to be more proactive, you may want to follow the advice of Dr. Giovannucci: “Given the strong (albeit unproven) suggestion of benefits at blood levels of at least 30 to 40 ng/ml, I see little downside for higher intakes (in the range of 1,000 to 2,000 IU a day), especially given the low likelihood of harm, and especially for groups at risk for vitamin D deficiency.”

Since 2008, we have suggested 800 to 1,000 IU of supplemental D a day—more if you have osteoporosis and/or your doctor says your blood level is low. We think that is a sensible middle ground.